This application discloses an invention which is related, generally and in various embodiments, to a Modular Multilevel Converter (M2LC) system and a method for controlling the M2LC system.
FIG. 1 illustrates an exemplary Multilevel Modular Converter (M2LC) system. The M2LC system includes a plurality of two-level M2LC cells (subsystems) arranged as output phase modules (e.g., Phase A, Phase B and Phase C), where each output phase module includes a plurality of series-connected two-level M2LC cells. The representative M2LC cell shown in FIG. 1 is a two-level M2LC cell which includes two switching devices, two diodes, a capacitor and two terminals. The two switching devices can be controlled such that one of two different potentials (e.g., zero volts or Vcap) may be present across the two terminals. As shown in FIG. 1, the respective output phase modules are arranged into a positive arm (an “N” level positive arm) connected to a positive DC bus (+ Bus) and a negative arm (an “N” level negative arm) connected to a negative DC bus (− Bus), where “N” equals the number of two-level M2LC cells in a given arm. The “N” two level M2LC cells produce N+1 arm voltage levels with respect to the positive or negative DC bus. For the Phase A positive arm, the individual M2LC cells may be designated as 0 pA, 1 pA and (N−1)pA. Similarly, for the Phase A negative arm, the individual M2LC cells may be designated as 0 nA, 1 nA and (N−1)nA. For a given output phase module, the positive and negative arms may be separated by an inductor.
The M2LC system is a relatively new voltage source bridge topology with performance similar to that of so called Cascaded H bridge topologies in regards to output voltage quality and availability, but without the need of being tethered to a complicated rectified multi-winding transformer. The resultant DC bus of the M2LC system however is unlike traditional voltage source converters in that currents that flow are continuous and the DC bus itself in immune to high inductance, resonance, and catastrophic bus fault conditions since the energy storage is resident to each series connected M2LC cell.
For the M2LC system shown in FIG. 1 (supplied from a DC source), the general control or modulation goal is to control the switching devices in the M2LC cells to produce the desired output voltage so that the sum of the M2LC cell output voltages in any positive or negative arm in a given output phase module always sums to the VDC supply voltage.
Existing M2LC systems typically size the inter-arm inductor sufficiently large (typically 3-5% of system size) to filter unwanted current harmonics (>=2nd harmonic) which are produced in the arms during the modulation to develop the desired fundamental output voltage. These large inductors are typically made from standard electrical grade steel and thus are usually large and heavy as well as possess significant magnetic and conductor losses. Also, a relatively low resonate frequency results in relation to their high value of inductance in resonance with the effective value of phase capacitance which is formed by the series connection of the filter capacitors of the M2LC cells. Due to switch function, the value of this phase capacitance is constant regardless of operating point and depends on the number of series M2LC cells together with the value of the filter capacitance. The switch function of a given M2LC cell shown in FIG. 1 is a function which represents a value “1” when the M2LC cell produces a voltage of “Vcap” between its two output terminals and a value “0” when the M2LC cell produces a short circuit condition between its two output terminals.
As a result, the value of this resonance has typically been close to or below both the operating output frequency and switching frequency of the M2LC cells. This requires the need for control systems to control the average value of the capacitor voltages over time and to control the potential for resonate conditions in the arm currents which can be excited by the operating and/or switching frequency of the M2LC cells. Additionally, these low resonate frequencies make it difficult and in-effective to control the ripple voltage of the cell filter capacitors at low operating frequencies and high output currents which is a condition usually required for most motor drive applications of the M2LC topology.
Traditional M2LC topologies size the inductor large enough to filter a majority of the harmonic current generated in the arm but as a result cause the resonate frequency of the phase or arms to be much lower than the switching frequency of the M2LC cells in the phase. As described hereinafter, this can be shown to cause a large component of the fundamental output current to flow in the cell filter capacitors and hence produce very large values of capacitor voltage ripple to occur at low operating frequencies.
One method suggested recently to control these high ripple voltages has been to add a common mode signal to the reference signals generating the desired output voltage. With low values of resonance, this common mode signal must also be as low or lower in frequency to have any meaningful effect. This signal also significantly interferes with the quality of the desired output voltage waveform when it is added or injected to limit this ripple voltage. Also, the ability to trade-off or control the ripple voltage on the filter capacitors of the M2LC cells at low output frequencies with the need to develop and control the magnitude of the desired output voltage as the desired output frequency increases can only be controlled by the magnitude of the added or injected common mode signal. This type of compensation can be very nonlinear, significantly affect the desired value of fundamental output voltage and introduce significant output distortion.